1. Field of the Invention
The present disclosure relates to fuel vapor processing systems, and in particular to fuel vapor processing systems for vehicles, such as automobiles, in which fuel vapor produced in a fuel tank is adsorbed by a canister and the adsorbed fuel is then purged into an intake air passage of an internal combustion engine.
2. Description of the Related Art
A vehicle equipped with an internal combustion engine (referred to as “engine”, hereafter) is provided with a fuel vapor processing system for inhibiting fuel vapor produced in a fuel tank from flowing into the atmosphere. For example, Japanese Laid-Open Patent Publication No. 2004-156499 discloses a fuel vapor processing system having an adsorbent canister capable of adsorbing fuel vapor, a tank passage communicating between the canister and a fuel tank, a purge passage communicating between the adsorbent canister and an air intake passage of an engine, an atmospheric passage communicating between the adsorbent canister and the atmosphere, a purge control valve provided in the purge passage, and a control unit for controlling the purge control valve. Fuel vapor produced in the fuel tank is introduced into the canister through the tank passage so as to be adsorbed by the adsorbent of the canister. Then, air is introduced into the canister through the atmospheric passage by utilizing negative pressure generated in the engine in order to desorb the fuel vapor from the canister and to purge it into the air intake passage. Thus, it is able to restore the adsorbing function of the canister and to use the fuel vapor adsorbed within the canister for burning in the engine.
The fuel vapor processing system of the aforementioned Publication No. 2004-156499 includes an electromagnetic closing valve capable of opening and closing the tank passage and a pressure sensor for detecting the inner pressure of the fuel tank (hereinafter called a tank internal pressure). The electromagnetic valve is opened for reducing the tank internal pressure to be less than a predetermined pressure, so that the tank internal pressure can be released.
However, if the electromagnetic closing valve is opened to release the tank internal pressure when the tank internal pressure is high or when the concentration of fuel vapor within the fuel tank is high, it may be possible that a part of the fuel vapor is discharged from the canister to the atmosphere via the atmospheric passage. This problem may occur in particular in the case that a relatively large amount of fuel vapor is still remained within the canister without being desorbed.
Therefore, there has been a need in the art for a fuel vapor processing system that can improve desorption of fuel vapor from canister.